When you have multiple Breakneck Mechs out at the same time, does it increase from Deflect 1 to Deflect 2 and so on?

Yes, multiple Breakneck Mech units on the board will cause their Deflect effects to stack.

According to Rule 809.2: "If a Unit has Deflect, or has been granted Deflect, and is granted Deflect by an additional source, the Deflect Value of all granted Deflect keywords is summed."

Since Breakneck Mech grants "Deflect" (which defaults to a value of 1 per Rule 809.1.b.3), having two copies of the card in play results in your other Mechs having Deflect 2. An opponent must pay 2 additional Power of any domain to target those units with a spell or ability.

Note on Ganking: While the Deflect stacks, the Ganking keyword does not. Per Rule 736.2, multiple instances of Ganking are redundant, so having multiple Breakneck Mechs provides no additional benefit to movement beyond what a single instance provides.